1. Field of the Invention
The present invention relates generally to near field optical microscopy, and in particular to a near-field optical microscope with an infrared fiber probe.
2. Description of the Related Art
For a standard microscope objective lens with a circular aperture, the diffraction of light through the aperture limits the smallest size that can be resolved by the objective. This diffraction is governed by the wavelength of light and the numerical aperture (half the inverse of the f-number) of the objective: ##EQU1##
where D is the diameter of the Airy disk (the minimum resolvable spot), .lambda. is the wavelength of the light that illuminates the subject, and NA is the numerical aperture of the lens, which must fall between 0 and 1. For a given wavelength, D is obviously smallest when the numerical aperture is closest to one. For real objectives a numerical aperture of one is physically impossible, but if we use this as the limiting value, then the minimum resolvable spot still works out to about 20% larger than the wavelength of the light used. For a microscope operating in the infrared (about 2-12 .mu.m), this means that the minimum resolvable spot will be much larger than one micrometer.
This limitation arises because the image is formed in the far field, at distances much greater than the wavelength of the light, after Fraunhofer diffraction has occurred. If, on the other hand, light is collected and an image formed in the near field, at distances shorter than a wavelength, the resolution is limited primarily by the size of the aperture used for the collection. This is the principle behind the scanning near field optical microscope (SNOM). The aperture can be of any kind, but due to the requirements for precise probe-to-specimen distance regulation, probes with very sharp tips and small cross-sectional area, like wires or optical fibers, are ideally suited to this application.
By tapering an optical fiber down to a very small tip diameter, it is possible to achieve resolution much smaller than the diffraction-limited spot size. Using a variety of methods such as etching or heat tapering, tips have been fabricated which yield optical resolution on the order of 50 nm. Aided by the superior resolution of these probes, investigations have been made of submicron implanted regions on semiconductor wafers, individual molecules in an organic film, and a host of other biological and inorganic specimens. A variety of permutations on the basic SNOM technique allows for even greater resolution or special sensitivity. Probes utilizing the magneto-optic Kerr effect (MOKE) have been used to image magnetic domains with 10 nm resolution. SNOM probes can also be used in atomic force (AFM) mode and as a type of "tunneling" optical probe, and also provide species identification which is unavailable from the STM and SFM (AFM) techniques. Spurred by the push for smaller linewidths in semiconductor fabrication, SNOM probes have been used to create near-field lithographic patterns using direct writing with ultraviolet beams. The demand for imaging capabilities from near-field optics has also led to the creation of arrays of SNOM probes and video-rate scanning techniques.
Though many applications such as biological imaging and semiconductor wafer inspection would benefit substantially from infrared optical probes with SNOM resolution, little work has been done in this area, and even less with optical transmission, reflection or excitation probes. Several of the types of fibers that transmit in the infrared, such as fluoride glass fibers and sapphire fibers, are unsatisfactory, due to transmission limitations in the IR. On the other hand, chalcogenide and chalcohalide glasses transmit well in the 2-12 .mu.m region.